It is known to control sheddable loads during a demand period in accordance with a priority table in order to increase or decrease the power drawn from the power system by the industrial equipment of a plant and stay within limits above, or below, a projected power demand limit to be reached at the end of the period. See for instance U.S. Pat. No. 3,522,421 of W. H. Miller and No. 3,872,286 of R. E. S. Putman.
It is known from the aforementioned Putman patent to control loads during the billing period so as to maintain the power demand at the end of the period close to a limit, or target, assigned by the user at the beginning of the period. In the process, loads are shed, or switched on, in accordance with a table of priorities while constraints imposed on certain loads, which are exercised before any control decision, affect the actual consumption of energy as well as the availability for control during the period. The target having been assigned by the operator, the power demand control system automatically and constantly, during the demand period, determines an error over the target from an instantaneous computation based on the wattage at the moment. A residual error is also established on the basis of a selected load to be shed, or switched on, depending on the sign of such error. The residual error leads to either actual switching or a decision to pursue the search for a load among the available loads in the order of priorities. Control during the period is by trial and error loading by successive approximations to an actual demand at the end of the period which is sufficiently close to the intended power demand limit imposed by the operator.
It is known from Iron and Steel Engineer of June 1975, pages 32-38, entitled "Power Demand Control -- Inland's No. 1 Electric Furnace Shop" by Ronald A. Balka and Floyd E. Demmon to calculate the rate of power consumption and to project on such basis the power usage to the end of the demand period and in so doing to take into account the rates computed for several prior demand pulses as well as the total demand pulses accumulated, thereby to keep the demand as close as possible to the target. A final demand which is too high would be costly and a final demand which is too low would be at the expense of the production capability of the plant.
It is also known from the Balka and Demmon article to change the load priorities scheduled by the computer program from demand period to demand period so that actual consumption follows more exactly the maximum power contract while satisfying the plant consumption requirements. This method amounts to imposing maximum power contract as a target to the system. The method, however, fails to seek as a target for the demand power demand at the end of the period a demand which is the closest to actual needs.
In practice, the loads in the plant are operated in accordance with production needs. This is the case for the ovens and furnaces, for instance. Some loads, though, merely assist in the production activity. These are for instance the fans and air conditioning units which operate within set constraints. All the loads of the plant, with or without any interruption, tend during the production activity to be ON and increase the demand. Therefore, the target assigned by the operator generally represents an upper limit to the power demand, not to be exceeded, and therefore requiring loads to be switched OFF, or interrupted with interruptions to be repeated, or lengthened in order to stay within such upper demand limit. If the demand limit has been placed too low in order to follow rigidly the requirement of cost reduction with the bills to the utility company, the consequence may be a curtailment of the overall activity in the plant by reducing the allocations of energy to the services supporting the production activity, and to the production units as well. Therefore, it is desirable to place the demand limit at an optimum level compatible with production requirements.
If the target must not be set too low since this would curb the production in the plant, it must not be set too high either since it would no longer effectively prevent an excessive demand. Therefore, it is desirable to establish the target for any billing period at an optimum level between too high and too low thereby to follow with good approximation the particular needs of the plant for the particular period.
However, the operator assigning a target to the power demand control system can only guess at the beginning of the billing period what the needs of the plants will be. He does not have enough information to optimize the balance between plant production requirements and power company demand billing.
It is observed here that the power demand limit imposed to the system at the beginning of the billing period is "ex ante," thus does not represent necessarily the best target for the on-coming billing period. It is also observed that a given target does not univocally determine how the actual consumption pattern will be in the plant since the latter depends upon the production priorities. These priorities are translated for control during the demand period by the priority table stored in the priority selector. Moreover, the calculations which are determining the error and are the basis for decision during the demand period depend upon the instantaneous combination of active loads and inactive loads, e.g., available and constrained loads. Correlatively, a change of targets may affect entirely the priority selection, and therefore the availability of loads under the system during the period. In the final analysis it is possible to allow a higher limit to the system in order to avoid unnecessary interruptions of production loads, while still deriving benefits from the power demand control system since the latter when in operation will assign temporary priorities, rotate the loads as scheduled and exercise the various constraints. However, an excessively high target will result in an excessively high bill from the power company for the actual demand.
It is in this context that an optimum target must be sought during each billing period so that savings can be obtained by limiting the power demand while at the same time allowing maximum consumption under a strict allocation as planned for the production in the plant.
The control system according to the present invention automatically adjusts power consumption in the plant during any particular billing period so as to hold the power demand through such billing period to an optimum demand limit.
The optimum target according to the present invention can best be explained in the context of the specification and drawings of U.S. Pat. No. 3,872,286 of Richard E. J. Putman issued on Mar. 18, 1975 and entitled "Control System And Method For Limiting Power Demand Of An Industrial Plant." Accordingly, the specification and drawings of U.S. Pat. No. 3,872,286 are hereby incorporated by reference for the purpose of providing a full description of the present invention.
As explained in the Putman patent, a production facility includes a plurality of loads which have received a special status in the order of production and a priority table has been established to allow them to be switched OFF or ON under power demand control. At a given moment, during the demand period, when decisions are to be made by the system, a load may be sheddable, e.g., have become available on a priority basis, or be no longer inhibited, e.g. an OFF time has lapsed to make it permissive. A load may be critical, e.g. under external requirements it may have become ON or OFF to follow local conditions overriding control by the system. Some loads never come under control of the system because they are "non-controllable" as the case is, for instance, for most lights, and more generally for the base load adding power constantly during the billing period. While at any instant during control operation the system receives a signal indicating the power in KW demanded from the utility company, this information represents a slope on the energy curve, and therefore it indicates power demand toward which the system is shooting at the end of the period. Control is effected by correcting the slope during the period so that the final demand will not exceed, or remain below, a target assigned to the plant power consumption at the end of the demand period. When attempting to control the demand to correct the slope by switching ON loads, there is a possibility that the slope increases to the point where it exceeds what is needed to stay on target.
In normal operation, if the target has been set right, the control system will cause loads to be switched ON, or OFF, so that under the priority selection tables the overall combination of loads in the plant is fully used as planned without overshooting or undershooting of the target.
If the target has been set too low, control will force many loads to be shut OFF, thus forcing the system down at the expense of production and against the plant planned requirements. If the target has been set too high, more loads will be switched ON and the consumption of power will be excessive; in fact, demand limiting will no longer be exercised.
The present invention is founded on a unique and new concept of a controlled optimization of the restrictions imposed by the demand limit during successive billing periods.
In the aforementioned Putman patent can be found the notion of using a pseudo-target for control during the billing period, which pseudo-target is abandoned and replaced by the real target before the end of the period is reached. This is the concept of a biased target which, as shown by Putman, is preferably under progressively reduced bias toward the end of the period. Such "moving" target, however, is used by Putman to allow heavier loads to be selected at the beginning of the period while lighter loads can be selected in the later part when the controlling margin is reduced.
In the Putman patent, also, is disclosed the assignment of a lower target in order to increase the rigidity of the system for better control, such lower target being automatically replaced by the assigned target later in the demand period when less rigidity has become necessary. This concept, however, does not suggest raising the target from one demand period to the next in order to reduce power demand while at the same time following the actual needs of the plant.
In accordance with the teaching of the Putman patent, whenever the demand is excessive, the lowest priority loads are turned OFF, while if the demand is insufficient to reach the target, the highest priority loads are turned ON. If the target is brought higher for the next demand period, assuming a Putman mode of control, more loads will be switched ON, or as a result of a lesser occurrence of excessive consumption anticipated, more loads of the lowest priority loads will remain ON. Thus by varying the target the system reaches a balance between production and charges for excessive demand.
However, whenever the production activity decreases substantially, the optimum target becomes too high. Although some rigidity in the control would be needed in order to save on the company bills, the control system no longer exerts control. It is proposed according to the present invention to lower the demand limit toward a new optimum in relation to the new level of production. More generally, the control system according to the present invention will be adapted to production changes downward as well as upward in order to maintain at all times, during the billing period an optimum target by allowing the demand limit imposed at the beginning of any demand period to float up and down in relation with fluctuations of the actual demand input to the plant facility.
An object of the present invention is to provide a system for monitoring and adjusting power demand within a demand period in which the power consumed during such demand period does not exceed a demand limit automatically selected by reference to actual demand within a prior demand period.
Another object of the present invention is to provide monitoring and adjusting of power demand within a demand period in accordance with an optimal demand limit.
Still another object of the present invention is to provide monitoring and adjusting of power demand of a plant for the billing period in accordance with a demand limit fluctuating up and down over the demand periods in response to actual demand.
It is a further object of the present invention to provide a power demand control system for imposing, during successive control periods, a power demand limit to a plurality of loads of different status and priorities of a production plant in which the imposed limit is automatically adjusted from one demand period to the next in order to follow closely the needs of the production plant.
Another object of the present invention is to provide a power demand control sytem for imposing during the demand periods a power demand limit to a plurality of loads of different status and priorities of a production plant in which the power demand limit at the beginning of each period is allowed to float automatically in accordance with the needs of the plant.